1. Field of the Invention
The present invention relates to a buffer device, and more particularly to a buffer device for a clock enable signal used in a memory device that is used when the memory device escapes from a self-refresh mode.
2. Description of the Prior Art
As generally known in the art, a volatile memory (hereinafter referred to as a memory device) is kept in an idle state before it performs a read/write operation. In the idle state, a clock enable (CKE) signal applied to the memory device is at a low level, i.e., in a disable state. Here, the clock enable (CKE) signal is used to control the operation of a buffer that receives a main clock being applied to the memory device. While the CKE signal is in a disable state, i.e., in an idle state, the memory device does not operate normally.
Also, while the CKE signal is kept in an idle state, the memory device periodically performs a refresh operation in order to prevent the loss of data stored in memory cells.
The refresh is performed in either a self-refresh mode or an auto-refresh mode.
The self-refresh is to refresh data periodically in the memory device itself, and the auto-refresh is to refresh data by a command of an external system. Hereinafter, the self-refresh operation will be explained as the refresh operation.
Generally, in the refresh state, a CKE buffer that receives a CKE signal does not perform a normal operation. In order for the memory device to escape from the self-refresh mode, a CKE signal of a high level is applied from the outside to the memory device and a separate buffer for recognizing the CKE signal is provided.
In the related art, a buffer for recognizing that the memory device escapes from the self-refresh mode is of a comparator type having a differential amplifier structure, and this comparator type buffer receives the CKE signal.
Typically, the comparator type buffer has two input terminals. A reference voltage of a specified level is applied from the outside to one input terminal, and a voltage applied through a CKE pin is applied to the other input terminal.
Here, the CKE pin is a clock enable pin used in the memory device. In an idle state, the CKE pin is kept at a low level while in an active state, it is kept at a high level.
In the idle state, the voltage of the CKE pin is of a low level, and the external reference voltage is kept at a specified voltage level. Accordingly, the comparator outputs a signal for keeping the self-refresh mode.
However, if the memory device enters into the active mode, the voltage of the CKE pin goes to a high level that is higher than the reference voltage level. Accordingly, the comparator output a signal for escaping from the self-refresh mode.
The above-described operation is possible on the assumption that the reference voltage applied to the CKE buffer is constant.
Recently, in order to reduce power consumption, some mobile products adopt a low level as the reference voltage level applied from the outside. In this case, the use of the conventional comparator type buffer as it is may cause malfunction of the memory device.
That is, if the reference voltage level is changed, the memory device may escape from the self-refresh mode at a undesirable time, and this may cause a problem in the operation of the memory device.